Vehicle powertrains include an engine and a transmission, wherein torque (or power) produced by the engine is transferred to the drive wheels through the transmission. In some powertrains, the engine is connected to the transmission via a clutch such as a bypass clutch on a torque converter equipped transmission or an input clutch on a dual clutch or automated manual transmission. Herein, the bypass clutch on a torque converter will be used to refer to either clutch type.
The slip of the bypass clutch is indicative of the amount of engagement of the torque converter. Operation of the bypass clutch at a high slip provides a large hydrodynamic coupling between the engine and the transmission which dampens driveline disturbances caused by torque disturbances from the engine, but reduces energy efficiency of the vehicle. Operation of the bypass clutch at a low slip provides a small hydrodynamic coupling between the engine and the transmission which does not attenuate the driveline disturbances, but provides improved energy efficiency of the vehicle. As described, the clutch slip allows for a reduction in the amount of engine torsional disturbance that reaches the transmission input and ultimately the vehicle occupants. The driveline disturbances are a measure of noise, vibration, and harshness (NVH) effects in the vehicle powertrain. Thus, while a large clutch slip reduces the NVH effects, it increases fuel consumption.
An off-line process which includes evaluation by vehicle NVH experts has been used to determine a desired amount of clutch slip (corresponding to a tolerable amount of NVH) for a given vehicle operating state. This process develops tables of allowable clutch slip versus various vehicle operating states such as engine speed, engine torque, gear selected, and temperature. This procedure is based on audible and tactile inputs to the driver.
Other NVH methods use measured acceleration of the transmission input shaft. The input shaft acceleration is felt to be the controlling factor of NVH level by some in the NVH community and is used as the quantitative underpinning of a qualitative process. Tables are built to relate acceptable NVH level to transmission input shaft acceleration, and transmission input shaft acceleration versus clutch slip.
The tables developed from the noted processes do not take into account all possible contributors to the excitation, nor do they take into account vehicle aging. As such, the tables may be inaccurate leading either to unacceptable NVH level or excessive fuel consumption.